8362
J. Phys. Chem. B 2009, 113, 8362–8368
Vapor Pressure Measurements by Mass Loss Transpiration Method with a Thermogravimetric Apparatus R. Viswanathan,* T. S. Lakshmi Narasimhan, and S. Nalini Fuel Chemistry DiVision, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603 102, India ReceiVed: January 29, 2009; ReVised Manuscript ReceiVed: April 30, 2009
Thermobalances are used for equilibrium vapor pressure measurements based on both effusion and transpiration methods. In the case of the transpiration method, however, despite the numerous advantages a thermogravimetric apparatus can offer, it is not as widely used as is the conventional apparatus. In this paper, the difference that can exist in the vapor phase compositions in an effusion cell and in a transpiration cell is shown first with two examples. Subsequently, how a commercial thermobalance was utilized to perform transpiration experiments that conform to the basic principle of the transpiration method and yield vapor pressures consistent with the Knudsen effusion mass spectrometric method is described. The three systems investigated are CsI(s), TeO2(s), and Te(s), each known to vaporize congruently, but in different manner. A critical analysis was performed on the information available in the literature on transpiration measurements using thermogravimetric apparatuses, and the salient findings are discussed. Smaller plateau regions than with conventional transpiration apparatuses and the lack of evidence for perfect transpiration conditions in some transpiration thermogravimetric investigations are shown with a few examples. A recommendation is made for the use of the rate of mass loss versus flow rate plot to ascertain that the usual apparent vapor pressure versus flow rate plot corresponds to a meaningful transpiration experiment. 1. Introduction Any substance will vaporize if the temperature is high enough. Knowledge of vaporization rates even at ambient temperatures is required for some organic compounds, which might pose health risks.1 The most readily conceivable consequence of vaporization under dynamic conditions is that the substance would incur mass loss. Thus, thermogravimetry, a method that will yield information on mass loss as a function of time as well as temperature, is often employed for conducting vaporization studies at temperatures of interest. The objective of a vaporization study usually is the measurement of equilibrium vapor pressures and subsequent derivation of various thermodynamic quantities employing the equations deduced from basic thermodynamic laws. How one deduces vapor pressure from the rate of mass loss depends on which method one uses: effusion or transpiration. Although the effusion method is very useful for measuring low vapor pressures (
